The present invention relates in general to systems for digitally representing analog waveforms and in particular to a system for processing digitized waveform data to produce a vector sequence representing the waveform.
Digital storage oscilloscopes typically convert a waveform into a series of digital values representing the magnitude of the waveform at regular sampling intervals, the digital data being stored sequentially in a random access acquisition memory as the waveform is sampled. Vector cathode ray tube oscilloscopes typically utilize the stored data to establish a series of vector coordinates on a cathode ray tube screen, recreating the analog waveform by drawing vectors (i.e. straight lines) from point-to-point on the screen. The accuracy of the representation improves with sampling rate since the acquired data will be more likely to reflect high frequency phenomona in the waveform. However a higher sampling rate requires a larger acquisition memory to store the acquired data. A larger acquisition memory is also required if a longer waveform is to be sampled and displayed with the same accuracy.
One method of improving accuracy without increasing the size of the acquisition memory involves sampling the waveform at a high sampling rate but storing in the acquisition memory only the minimum and maximum data values acquired during successive sampling intervals. The waveform is then reproduced using these minimum and maximum values as the vector coordinates for the vectorized waveform display, the vector coordinates being plotted in minimum-maximum-minimum-maximum fashion.
The accuracy of the representation is improved if the maximum and minimum of each sampling period are plotted in the order in which they occur. In the past this has been accomplished by employing a maximum/minimum detector having circuitry which keeps track of the order in which the maximum and minimum occur during each sampling period and then transfers the data to the acquisition memory in the proper order at the end of each sampling period. However the proper ordering of the data prior to storage in the acquisition memory requires additional circuitry in the minimum/maximum detector and can require additional steps during data acquisition which can affect the maximum sampling rate.
Even with the maxima and minima properly ordered, the waveform reproduction is distorted by the placement of the maxima and minima. For instance in a smoothly rising waveform, the minimum occurs at the beginning of each sampling period while the maximum occurs at the end of each period. But normally the vector endpoints are plotted at regular intervals on the CRT screen, the minimum point for each sampling period being plotted at the one quarter period point and the maximum point being plotted at the three-quarter period point. In this way the waveform is represented by a series of vectors connected end-to-end, each having the same horizontal displacement and a vertical displacement according to the difference between the successive maximum and minimum levels. In a smoothly rising waveform, the maximum for one period would be substantially equal to the minimum of the next period. Thus a horizontal vector one half-period long would connect the maximum of one sampling period to the minimum of the next and a smoothly rising waveform would therefore be represented as an increasing saw-tooth, or stepped waveform. Distortion would also occur on the falling portion of the waveform.